The diffusiophoresis of a charge-regulated spherical particle normal to two parallel disks as a response to an applied uniform electrolyte concentration gradient is modeled theoretically. The fixed charge on the particle surface comes from the dissociation/association reactions of the functional groups, yielding a charge-regulated surface, which simulates biological cells. Numerical simulations are conducted to examine the behavior of a particle under various conditions: the parameters considered in the simulation include the thickness of the double layer, the charged conditions on the particle surface, the relative size of the particle, and the particle-disk distance. Because the diffusiophoretic mobility of a particle can be dominated by chemiphoresis, electrophoresis, or osmotic flow, the diffusiophoretic behavior of the particle is complicated. For instance, the diffusiophoretic
mobility may have two local maximums and a local minimum as the thickness of the double layer varies. This behavior is of practical significance if diffusiophoresis is adopted as a separation operation or as a tool to characterize the surface properties of a particle.